Deerfield Academy Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deerfield Academy Planetarium clear sky chart from 2005-11-06 to 2026-01-17.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	17% 7% 3 7% 5% 4 5% 4 6% 7% 37%	127228
	Night	17% 8% 3 7% 4 4 4 3 5% 6% 39%	124454
Transparency	Day	47% 4 11% 22% 14% 2	130735
	Night	46% 4 12% 21% 14% 2	129034
Seeing	Day	23% 15% 29% 20% 9% 3	16377
	Night	22% 12% 30% 28% 8% 1	25451

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 6% 4 12% 4 6% 5% 4 6% 6% 34%	3677
	11th to 20th	Day	13% 9% 4 9% 4 4 4 4 5% 6% 39%	3618
	21st to end	Day	15% 9% 4 7% 5% 4 4 3 4 6% 39%	3960
February	1st to 10th	Day	15% 9% 3 6% 6% 6% 6% 4 6% 7% 32%	1845
	11th to 20th	Day	13% 8% 3 8% 6% 4 4 5% 5% 6% 37%	1948
	21st to end	Day	18% 10% 2 8% 5% 5% 5% 2 5% 5% 35%	1650
March	1st to 10th	Day	20% 11% 4 7% 5% 3 4 3 5% 5% 34%	4476
	11th to 20th	Day	16% 7% 2 7% 5% 4 5% 4 6% 6% 39%	4645
	21st to end	Day	23% 9% 2 6% 4 4 3 3 4 6% 36%	5970
April	1st to 10th	Day	19% 8% 3 7% 6% 4 3 3 5% 6% 37%	2400
	11th to 20th	Day	16% 7% 3 6% 5% 4 5% 3 6% 6% 38%	2374
	21st to end	Day	19% 7% 2 7% 4 5% 4 5% 5% 6% 36%	2550
May	1st to 10th	Day	31% 8% 2 6% 4 4 4 4 5% 6% 23%	5886
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 5% 6% 31%	6216
	21st to end	Day	18% 7% 2 6% 4 4 4 4 7% 8% 34%	6949
June	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 4 7% 9% 34%	2800
	11th to 20th	Day	16% 6% 2 5% 5% 5% 5% 5% 6% 8% 38%	2800
	21st to end	Day	15% 7% 3 6% 5% 5% 5% 4 5% 8% 36%	2800
July	1st to 10th	Day	12% 7% 2 7% 5% 5% 6% 5% 8% 9% 34%	6360
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 4 7% 11% 37%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 7% 43%	6926
August	1st to 10th	Day	14% 6% 2 7% 5% 6% 5% 4 6% 9% 34%	4433
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 5% 6% 8% 37%	3961
	21st to end	Day	9% 5% 2 5% 4 5% 6% 5% 7% 10% 43%	4312
September	1st to 10th	Day	12% 5% 2 5% 5% 4 4 4 6% 7% 45%	2400
	11th to 20th	Day	13% 5% 3 4 3 3 4 3 5% 7% 50%	2254
	21st to end	Day	17% 5% 2 5% 3 3 4 4 6% 8% 42%	2200
October	1st to 10th	Day	21% 6% 2 6% 4 4 5% 3 4 5% 41%	2174
	11th to 20th	Day	14% 6% 2 5% 4 5% 4 4 5% 7% 43%	1950
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 6% 38%	1980
November	1st to 10th	Day	12% 7% 2 5% 4 4 3 3 4 6% 49%	1851
	11th to 20th	Day	16% 7% 3 4 4 3 5% 3 4 7% 43%	1890
	21st to end	Day	18% 9% 2 5% 4 3 4 4 5% 6% 39%	1819
December	1st to 10th	Day	16% 8% 3 7% 5% 5% 4 3 4 6% 37%	2680
	11th to 20th	Day	20% 8% 3 5% 4 3 3 3 4 5% 41%	3066
	21st to end	Day	18% 10% 3 7% 5% 4 4 3 3 7% 35%	4048
January	1st to 10th	Night	14% 6% 3 9% 4 4 5% 3 5% 7% 39%	6489
	11th to 20th	Night	12% 10% 3 11% 6% 5% 5% 4 6% 6% 34%	6400
	21st to end	Night	14% 10% 3 7% 4 4 4 4 6% 6% 37%	6996
February	1st to 10th	Night	14% 10% 3 6% 6% 5% 5% 4 6% 6% 36%	2548
	11th to 20th	Night	11% 9% 3 8% 5% 4 5% 4 5% 7% 38%	2338
	21st to end	Night	16% 8% 3 7% 5% 4 5% 4 4 6% 38%	1980
March	1st to 10th	Night	22% 8% 2 6% 5% 4 4 3 5% 7% 35%	5364
	11th to 20th	Night	15% 9% 3 6% 4 4 4 3 6% 6% 39%	5296
	21st to end	Night	23% 9% 3 6% 3 3 4 3 5% 5% 35%	4962
April	1st to 10th	Night	18% 9% 2 6% 6% 4 4 3 5% 6% 36%	2000
	11th to 20th	Night	19% 8% 2 5% 4 5% 4 4 5% 6% 39%	1975
	21st to end	Night	21% 7% 2 7% 3 3 3 3 5% 6% 40%	1850
May	1st to 10th	Night	33% 9% 3 6% 3 4 4 3 5% 6% 26%	4054
	11th to 20th	Night	17% 10% 4 9% 6% 5% 5% 3 6% 6% 29%	3720
	21st to end	Night	22% 7% 3 7% 4 4 4 3 6% 7% 33%	3926
June	1st to 10th	Night	17% 8% 2 7% 4 4 4 3 4 6% 41%	1600
	11th to 20th	Night	18% 6% 2 5% 5% 4 4 4 6% 7% 40%	1600
	21st to end	Night	15% 6% 2 5% 4 4 4 4 7% 7% 42%	1600
July	1st to 10th	Night	12% 6% 2 6% 5% 5% 5% 3 6% 9% 41%	3600
	11th to 20th	Night	12% 7% 3 6% 5% 5% 4 5% 7% 9% 38%	3600
	21st to end	Night	12% 5% 2 6% 3 4 3 3 4 6% 51%	3926
August	1st to 10th	Night	21% 6% 2 7% 4 5% 4 4 5% 7% 35%	2684
	11th to 20th	Night	13% 7% 3 8% 5% 4 4 3 6% 7% 39%	3069
	21st to end	Night	10% 4 1 4 3 4 4 3 5% 8% 54%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 48%	2000
	11th to 20th	Night	12% 5% 1 5% 3 3 4 3 4 6% 54%	2145
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 7% 44%	2200
October	1st to 10th	Night	20% 6% 2 5% 5% 3 3 3 5% 6% 43%	2224
	11th to 20th	Night	16% 5% 2 5% 3 3 4 3 6% 6% 45%	2449
	21st to end	Night	22% 8% 1 6% 4 4 3 3 4 6% 39%	2860
November	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 5% 6% 48%	2667
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 4 5% 44%	2730
	21st to end	Night	15% 8% 2 6% 4 4 4 4 6% 6% 42%	2798
December	1st to 10th	Night	17% 10% 3 6% 4 4 4 3 6% 6% 38%	4720
	11th to 20th	Night	24% 8% 2 6% 3 3 3 3 5% 5% 39%	5348
	21st to end	Night	19% 10% 4 8% 5% 3 4 3 4 5% 35%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	52% 3 9% 24% 10% 1	3678
	11th to 20th	Day	46% 4 10% 19% 17% 3	3648
	21st to end	Day	48% 3 8% 21% 18% 2	3960
February	1st to 10th	Day	49% 6% 13% 18% 13% 1	1845
	11th to 20th	Day	48% 5% 10% 20% 14% 3	1957
	21st to end	Day	51% 4 10% 20% 14% 2	1650
March	1st to 10th	Day	56% 2 11% 13% 10% 7%	4479
	11th to 20th	Day	45% 5% 15% 23% 9% 3	4645
	21st to end	Day	52% 2 5% 16% 19% 6%	5970
April	1st to 10th	Day	49% 4 9% 20% 13% 4	2400
	11th to 20th	Day	45% 6% 10% 19% 18% 2	2374
	21st to end	Day	48% 5% 9% 19% 16% 3	2550
May	1st to 10th	Day	61% 6% 10% 13% 9% 1	5897
	11th to 20th	Day	55% 4 13% 17% 10% 1	6220
	21st to end	Day	47% 6% 11% 23% 13% 1	6949
June	1st to 10th	Day	44% 7% 14% 22% 12% 1	2800
	11th to 20th	Day	43% 7% 14% 20% 14% 2	2800
	21st to end	Day	45% 7% 12% 21% 13% 1	2800
July	1st to 10th	Day	47% 5% 13% 21% 14% 1	6290
	11th to 20th	Day	43% 4 16% 24% 12% 1	6285
	21st to end	Day	41% 3 12% 27% 16% 1	6728
August	1st to 10th	Day	48% 5% 15% 21% 10% 1	4419
	11th to 20th	Day	44% 4 14% 28% 9% 1	4000
	21st to end	Day	36% 4 14% 29% 15% 2	4312
September	1st to 10th	Day	37% 6% 13% 26% 17% 1	2400
	11th to 20th	Day	36% 5% 9% 26% 22% 2	2254
	21st to end	Day	42% 5% 10% 23% 17% 3	2200
October	1st to 10th	Day	45% 2 8% 23% 20% 2	3538
	11th to 20th	Day	36% 3 9% 28% 22% 2	3153
	21st to end	Day	36% 3 12% 30% 15% 3	3180
November	1st to 10th	Day	39% 4 15% 25% 16% 2	1851
	11th to 20th	Day	42% 4 10% 22% 19% 3	1890
	21st to end	Day	46% 5% 14% 24% 10% 1	1819
December	1st to 10th	Day	49% 3 14% 25% 8% 1	2680
	11th to 20th	Day	48% 3 12% 20% 17% 2	3066
	21st to end	Day	52% 4 7% 19% 14% 4	4048
January	1st to 10th	Night	47% 3 11% 25% 11% 2	6489
	11th to 20th	Night	51% 4 13% 17% 13% 3	6445
	21st to end	Night	48% 3 11% 16% 17% 4	6996
February	1st to 10th	Night	48% 5% 13% 22% 10% 2	2548
	11th to 20th	Night	46% 5% 12% 20% 14% 3	2340
	21st to end	Night	48% 4 10% 22% 11% 4	1980
March	1st to 10th	Night	51% 4 10% 16% 15% 4	5374
	11th to 20th	Night	45% 5% 18% 20% 10% 2	5297
	21st to end	Night	52% 3 5% 14% 22% 4	4962
April	1st to 10th	Night	50% 4 9% 20% 15% 2	2000
	11th to 20th	Night	47% 6% 9% 20% 18% 1	1975
	21st to end	Night	46% 5% 9% 20% 15% 5%	1850
May	1st to 10th	Night	60% 5% 12% 12% 9% 1	4060
	11th to 20th	Night	56% 6% 10% 16% 10% 2	3721
	21st to end	Night	51% 6% 13% 18% 11% 1	3927
June	1st to 10th	Night	46% 6% 11% 25% 12% 1	1600
	11th to 20th	Night	44% 6% 14% 23% 12% 1	1600
	21st to end	Night	41% 6% 14% 24% 14% 1	1600
July	1st to 10th	Night	42% 5% 15% 22% 16%	3560
	11th to 20th	Night	45% 5% 15% 21% 14% 1	3552
	21st to end	Night	36% 3 13% 30% 17% 1	3809
August	1st to 10th	Night	50% 5% 12% 21% 10% 1	2674
	11th to 20th	Night	45% 4 15% 25% 10%	3096
	21st to end	Night	30% 4 14% 35% 17% 1	3586
September	1st to 10th	Night	38% 6% 11% 27% 18% 1	2000
	11th to 20th	Night	33% 5% 10% 26% 23% 2	2145
	21st to end	Night	41% 5% 9% 25% 17% 3	2200
October	1st to 10th	Night	41% 3 13% 25% 16% 2	3640
	11th to 20th	Night	32% 4 10% 26% 25% 3	3985
	21st to end	Night	37% 3 13% 32% 14% 1	4610
November	1st to 10th	Night	39% 6% 10% 25% 17% 3	2667
	11th to 20th	Night	44% 5% 10% 22% 17% 3	2730
	21st to end	Night	43% 5% 15% 25% 12% 1	2798
December	1st to 10th	Night	49% 3 11% 27% 9%	4720
	11th to 20th	Night	50% 3 14% 15% 17% 2	5348
	21st to end	Night	53% 3 8% 20% 12% 4	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	26% 28% 24% 19% 4	220
	11th to 20th	Day	30% 25% 31% 9% 4 1	212
	21st to end	Day	24% 26% 30% 17% 4	220
February	1st to 10th	Day	32% 26% 31% 9% 1	347
	11th to 20th	Day	31% 24% 30% 13% 1	496
	21st to end	Day	30% 18% 32% 17% 3	414
March	1st to 10th	Day	31% 19% 32% 15% 2	500
	11th to 20th	Day	31% 21% 29% 14% 5%	500
	21st to end	Day	30% 23% 29% 11% 4 3	550
April	1st to 10th	Day	34% 22% 25% 12% 6% 1	500
	11th to 20th	Day	31% 21% 27% 14% 5% 1	492
	21st to end	Day	29% 18% 30% 16% 6% 2	500
May	1st to 10th	Day	33% 19% 26% 15% 6% 2	503
	11th to 20th	Day	23% 17% 35% 17% 6% 1	527
	21st to end	Day	19% 13% 31% 24% 11% 3	573
June	1st to 10th	Day	18% 13% 33% 23% 11% 2	530
	11th to 20th	Day	18% 13% 33% 24% 10% 2	530
	21st to end	Day	17% 14% 34% 20% 11% 4	530
July	1st to 10th	Day	17% 7% 32% 26% 14% 3	525
	11th to 20th	Day	13% 8% 28% 31% 17% 4	524
	21st to end	Day	16% 7% 32% 29% 14% 2	576
August	1st to 10th	Day	11% 7% 26% 32% 18% 5%	526
	11th to 20th	Day	14% 8% 29% 26% 17% 7%	519
	21st to end	Day	11% 10% 29% 29% 15% 6%	583
September	1st to 10th	Day	19% 6% 26% 24% 17% 7%	530
	11th to 20th	Day	16% 8% 26% 22% 17% 11%	530
	21st to end	Day	20% 9% 28% 24% 12% 7%	530
October	1st to 10th	Day	26% 11% 24% 19% 15% 5%	530
	11th to 20th	Day	21% 15% 28% 22% 11% 3	502
	21st to end	Day	29% 14% 24% 23% 8% 1	462
November	1st to 10th	Day	24% 13% 30% 23% 8% 3	425
	11th to 20th	Day	29% 18% 27% 20% 6%	430
	21st to end	Day	32% 17% 27% 19% 4 1	359
December	1st to 10th	Day	25% 22% 29% 18% 5% 1	220
	11th to 20th	Day	35% 21% 31% 11% 2	220
	21st to end	Day	30% 16% 34% 14% 5%	242
January	1st to 10th	Night	22% 16% 35% 25% 3	840
	11th to 20th	Night	27% 15% 31% 23% 3	825
	21st to end	Night	20% 16% 36% 24% 4	880
February	1st to 10th	Night	22% 16% 37% 24% 2	800
	11th to 20th	Night	22% 18% 38% 20% 2	783
	21st to end	Night	24% 17% 33% 23% 3	660
March	1st to 10th	Night	25% 16% 35% 21% 2	800
	11th to 20th	Night	23% 19% 35% 20% 4	800
	21st to end	Night	24% 17% 32% 21% 6%	880
April	1st to 10th	Night	26% 16% 32% 23% 4	800
	11th to 20th	Night	24% 18% 30% 22% 6%	791
	21st to end	Night	26% 15% 31% 22% 4	650
May	1st to 10th	Night	29% 13% 30% 24% 5%	600
	11th to 20th	Night	19% 13% 35% 27% 6%	461
	21st to end	Night	20% 7% 28% 33% 11%	433
June	1st to 10th	Night	19% 5% 28% 38% 10%	400
	11th to 20th	Night	19% 7% 30% 35% 9% 1	400
	21st to end	Night	16% 6% 27% 40% 11%	400
July	1st to 10th	Night	14% 4 25% 43% 14% 1	390
	11th to 20th	Night	17% 3 21% 41% 18% 1	388
	21st to end	Night	11% 4 23% 48% 14%	438
August	1st to 10th	Night	15% 2 21% 46% 16% 1	457
	11th to 20th	Night	14% 4 21% 40% 19% 2	711
	21st to end	Night	16% 4 21% 36% 21% 1	880
September	1st to 10th	Night	17% 4 25% 35% 18% 2	800
	11th to 20th	Night	15% 5% 21% 33% 22% 4	800
	21st to end	Night	19% 5% 25% 36% 14% 2	800
October	1st to 10th	Night	22% 9% 22% 30% 15% 2	800
	11th to 20th	Night	20% 10% 29% 30% 11%	800
	21st to end	Night	27% 10% 31% 26% 7%	880
November	1st to 10th	Night	19% 13% 30% 29% 8% 1	820
	11th to 20th	Night	22% 13% 32% 26% 6% 1	840
	21st to end	Night	22% 12% 30% 29% 6%	840
December	1st to 10th	Night	26% 13% 34% 22% 5%	840
	11th to 20th	Night	26% 13% 32% 25% 3	840
	21st to end	Night	28% 14% 32% 23% 3	924

Page generated at 2026-01-14 01:14:30. Version ac07

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